Working with binary in Python, Splitting numbers

5
votes

I have a module that takes 2 X 8bit numbers in Decimal format, with a specific structure

each number must start with the same 4 bits = 0011 followed be a varible 8 bits followed by 4 bits that are ignored so set to 0000

So to caculate the 16bit number is simple enough

the varible number * 16 will shift it the 4 bits to the left, and adding 12288 = 0011|000000000000 will give me the desired result.

so if my input number is 19 for example

19 X 16 + 12288 = 12592 = 0011000100110000

the next step is to split it in to two X 8 bit numbers

00110001 | 00110000 = 49, 48

how in python can i go from 12592 to 49,48 efficiently.

Never worked in binary in a script so all a bit new.

Cheers

3
pythonbinary
So, basically you are asking how, given a 16 bit number, you can get the number corresponding to the first and the last 8 bits? - tobias_k
yep :) that would be the simple way of saying it. - DevilWAH

3 Answers

8
votes

To get the first 8 bits, just shift it to the right by 8 bits. 

   0011000100110000 >> 8
==         00110001

To get the last 8 bits, mask it with 0b11111111, i.e. 255.

   0011000100110000
&  0000000011111111
-------------------
   0000000000110000

Code example:

>>> n = int("0011000100110000", 2)
>>> n
12592
>>> n >> 8, n & 255
(49, 48)

Alternatively, you could also just use divmod, but >> and & seem to be a bit faster.

>>> divmod(n, 256)
(49, 48)
2
votes

Make use of the bin built in function

def split16Bit(num):
    binary = bin(num)[2:].rjust(16, '0')
    return (int(binary[:8], 2), int(binary[8:], 2))
0
votes

Here's an example, not sure if it applies since you haven't provided example code.

from binascii import hexlify
from sys import version_info

if version_info >= (3, 0):
    return bin(int.from_bytes(text_data.encode(), 'big'))
else:
    return bin(int(hexlify(text_data), 16))

This was taken from code I already use in a one time pad encryption module i wrote. It converts strings just fine, heres the the entire method.

def _string_converter(self, text_data):
    """Takes a given string or file and converts it to binary."""
    if version_info >= (3, 0):
        return bin(int.from_bytes(text_data.encode(), 'big'))
    else:
        return bin(int(hexlify(text_data), 16))

As an example to how to convert a string into 16bit binary, it could be split into 8 bit segments, probably easiest with some list magic.